This invention relates to the combustion of fuels and more particularly to the efficient combustion of compressible fluid fuel.
Efficient combustion of liquid fuels usually involves creating fine liquid droplets or vaporizing the liquid to the gaseous state and thereafter mixing the fuel with air or oxygen in order to initiate and sustain combustion. As disclosed and claimed in our co-pending United States Patent Application Serial No. 08/992,983, now U.S. Pat. No. 6,010,544, the contents of which are incorporated herein by reference, an alternative approach involves forming a supercritical water/hydrocarbon fuel mixture. As taught in that patent, a water/hydrocarbon fuel mixture is heated and pressurized to a level at or above the critical point of the mixture. At critical conditions, the mixture is a homogeneous single phase that can be combusted in a more efficient manner and with considerably reduced undesirable emissions. As further taught therein, the critical temperature of the mixture is at or above approximately 363xc2x0 C. and the critical pressure is at or above 3000 psi. Optimal conditions are considered to be 390xc2x0 C. and 4000 psi.
As disclosed and claimed in our co-pending U.S. patent application Ser. No. 09/359,509, now U.S. Pat. No. 6,240,893, the contents of which are incorporated herein by reference, another alternative approach involves forming a sub-critical water/hydrocarbon fuel mixture which is at a pressure that is below the critical pressure charactistic of the mixture and is at a temperature that is at least the greater of about 250xc2x0 C. and the boiling point of water at the mixture pressure. As taught in that application, a water/hydrocarbon fuel mixture is pressurized to a level below the critical point of the mixture and heated to a temperature that is at least the greater of about 250xc2x0 C. and the boiling point of water at the mixture pressure. The specified sub-critical mixture provides a local environment of water molecules, tending to limit hydrocarbon polymerization and other undesrable side reactions and keeping the hydrocarbon from precipitating from the mixture. At these specified sub-critical conditions, combustion of the mixture provides a xe2x80x9ccleanerxe2x80x9d combustion with considerably reduced particulate matter and oxides of nitrogen emissions compared with that which results from the combustion of otherwise comparable water-hydrocarbon mixtures at temperatures below the boiling point of water at the sub-critical pressure employed.
In some applications, the high pressure and temperature of the supercritical mixture or the above-specified subcritical mixtures are directly suitable for injection into a combustion chamber. In other applications, the high pressure of these water/hydrocarbon fuel mixtures result in subsequent fuel/air flow velocities higher than the flame speed of the combustible mixture. Flow velocities which far exceed the flame speed lead to difficulties with ignition and flame stabilization. Thus, for applications such as nearly atmospheric pressure oil burners the fuel/air mixture flow velocity must be reduced by lowering both the pressure and velocity.
A valve could be used to lower the flow velocity since it provides for a variable size orifice, but some of total pressure ahead of the valve is recovered in the velocity downstream of the valve and a valve also introduces increased thermal mass and dwell time. Furthermore, after the fluid passes through the valve its temperature and pressure change dramatically and the fluid""s metastable condition can be adversely affected by these changes in the length between the valve and the combustion process. Other solutions to for lowering flow velocity are thus needed.
In one aspect, the fuel system of the invention includes structure containing a mixture of water and hydrocarbon fuel in which the mixture is at or above its critical temperature and pressure such that the mixture is a homogeneous single phase. Apparatus is provided for rapidly reducing the total pressure of the mixture prior to delivery to a combustion chamber. In another aspect, the invention is a process for combusting a hydrocarbon/water mixture including the steps of producing a mixture of water and hydrocarbon fuel at or above its critical point such that the mixture is a homogeneous single phase and rapidly reducing the pressure of the mixture without an excessive change in the velocity. Thereafter, the reduced pressure mixture is delivered into a combustion chamber. In this aspect, it is preferred that the mixture be maintained at a temperature above 363xc2x0 C. and at a pressure above 3000 psi before reduction. It is preferred that the pressure reduction occur within a time period less than 1 millisecond prior to being delivered to the combustion chamber and that the pressure be reduced to below 200 psi for atmospheric combustion. The temperature of the supercritical mixture may be in the range of 363xc2x0 C.-450xc2x0 C. and in a pressure range of 3000 psi 4500 psi. It is also preferred that the pressure reduction take place within a time period in the range of 0.1-2 milliseconds prior to being delivered to a combustion chamber and that the total pressure be reduced to a range of 2 to 10 times the pressure in the combustion chamber.
In another aspect, the fuel system of the invention includes structure containing a sub-critical water/hydrocarbon fuel mixture which is at a pressure that is below the critical pressure characteristic of the mixture and is at a temperature that is at least the greater of about 250xc2x0 C. and the boiling point of water at the mixture pressure. Apparatus is provided for rapidly reducing the pressure of the mixture prior to delivery to a combustion chamber. In another aspect, the invention is a process for combustion of a sub-critical hydrocarbon/water mixture of the above-specified characteristics and rapidly reducing the pressure of the mixture. Thereafter, the reduced pressure mixture is delivered into a combustion chamber. In this aspect, it is preferred that the mixture be maintained at a temperature that is between about 25xc2x0 C. and about 100xc2x0 C. greater than the boiling point of water at the mixture pressure. It is preferred that the pressure reduction occur within a time period less than 1 millisecond prior to being delivered to the combustion chamber and that the pressure be reduced to below 200 psi for combustion at up to 10 atmospheres of pressure. It is also preferred that the pressure reduction take place within a time period in the range of 0.1-2 milliseconds prior to being delivered to a combustion chamber and that the pressure be reduced to a range of 200-500 psi. for combustion at 10 to 20 atmospheres
Apparatus is provided for rapidly reducing the total pressure of the mixture prior to delivery to a combustion chamber. In another aspect, the invention is a process for combusting a hydrocarbon/water mixture including the steps of producing a supercritical or specified sub-critical mixture of water and hydrocarbon fuel and rapidly reducing the pressure of the mixture. Thereafter, the reduced pressure mixture is delivered into a combustion chamber. In this aspect, it is preferred that a supercritical mixture be maintained at a temperature above 363xc2x0 C. and at a pressure above 3000 psi. It is preferred that the pressure reduction occur within a time period less than 1 millisecond prior to being delivered to the combustion chamber and that the pressure be reduced to below 200 psi for combustion at 1 to 10 atmospheres. The temperature of the supercritical mixture may be in the range of 363xc2x0 C.-450xc2x0 C. and in a pressure range of 3000 psi-4500 psi. It is also preferred that the pressure reduction take place within a time period in the range of 0.1-2 milliseconds prior to being delivered to a combustion chamber and that the pressure be reduced to a range of 200-500 psi for combustion at 10 to 20 atmospheres.
In another aspect, the apparatus for reducing the total pressure of the supercritical or specified sub-critical mixture comprises at least two closely spaced apart constant enthalpy expansion sections, each section having at least one orifice with orifices in adjacent sections being non-coaxial. Each section defines an enclosed volume and the size of the orifices increases in a flow direction in adjacent sections. It is preferred that the orifice size at each section be selected to provide sonic or choked flow at each section. In a preferred embodiment, the first section includes a single orifice and subsequent sections include at least two orifices and preferably three orifices. Also in a preferred embodiment the pressure downstream of the orifice is reduced to approximately one-third of the upstream pressure at each section. In this embodiment, the volumes between orifices serve to dissipate velocity of expansion by local shock waves, thus reducing total as well as static pressures.
In yet another aspect, the apparatus for reducing the pressure of the supercritical water/hydrocarbon fuel mixture includes three closely spaced apart constant enthalpy expansion sections for receiving the mixture. A first section includes a single orifice located on a central axis of the first section and a second section includes three orifices located off the central axis. A third section includes six orifices arranged on a substantially spherical surface. This device provides the mixture at pressures near 1 atmosphere and velocity suitable for mixing with air and combusting.
This invention relates to the combustion of compressible fluid fuels, including but not limited to the above-described supercritical or sub-critical water/hydrocarbon fuel mixtures. Compressible fluid fuels (e. g., methane or xe2x80x9cnatural gasxe2x80x9d) are at least in part gas like (i. e., compressible) at ambient conditions and typically are used by being injected into the cylinder of a reciprocating engine at near top dead center. The space available for this injection is so limited that the gaseous fuel must be delivered under high pressure. Injection of this gaseous fuel under high pressure directly into a cylinder can result in a gas jet, which transits the cylinder too rapidly to entrain the appropriate amount of air. Delivery of the gaseous fuel to the cylinder from the injector through apparatus of this invention with more than one set of orifices in series can eliminate or minimize this problem.
The multisection cascade orifice nozzle provides for relieving the momentum of the high-pressure jet while maintaining the intimate mixing of water and oil under the supercritical conditions. The invention produces the correct fluid velocity distribution by interacting the flow from the final orifice it with the combustion air mass. The momentum exchange to the air also includes mixing so that the proper mixture of air and fuel is obtained locally.
In contrast, the mixture issuing from a single orifice is moving so fast that it entrains too much air and is too lean for stable combustion. Pressure reductions from the 4000-psi range down to the 100-psi range can be achieved within a length of approximately 1-inch and the time spent in this volume can be reduced to the millisecond range for high flow rates. Metastable conditions at reduced pressures can be sustained long enough to begin and maintain stable combustion.